2 Answers
2

Just figured this out. It looks like the rules for magic armor changed with HotFL.

The masterwork bonus is automatically applied no matter what type of armor you are using. You don't need Layered Plate or Rimefang Plate to get those masterwork bonuses, they are automatically rolled in to the native magic item.

The masterwork armors still exist if you want to change things up (like sacrificing some AC to gain in your NADs).

What happened here is that unarmored characters and characters wearing light armor (cloth, leather and hide) use their Dex skill to determine AC. However, wearers of Mail, Scale and Plate use only the armor modifier.